When your biceps muscle in your higher arm contracts, it pulls your lower arm in to your shoulder. when it relaxes, your biceps cant push your arm back out. and straightens your arm out.
When your biceps muscle in your upper arm contracts, it pulls your lower arm in towards your shoulder. However, when it relaxes, your biceps cannot push your arm back out. To do this, your triceps muscle, on the underside of your upper arm, contracts and straightens your arm out.
